1. Field of the Invention
The present invention relates to an oil pump mechanism, and more particularly, to an oil pump mechanism incorporated in a chain saw to lubricate a saw chain of the chain saw.
2. Description of the Related Art
A chain saw comprises a main body internally having a two-cycle internal combustion engine, a guide bar extending frontward from the main body, and a saw chain to be driven along a guide groove around the guide bar. The chain saw is driven by a driving force transmitted from the engine through a centrifugal clutch provided with a clutch shoe connected in driven relation to a crankshaft of the engine and a clutch drum disposed coaxially around the crankshaft.
There has been known one type of chain saw equipped with an oil pump mechanism designed to be activated in synchronization with the centrifugal clutch so as to supply a lubricating oil to the saw chain, as disclosed, for example, in U.S. Pat. No. 3,448,829 and Japanese Patent Laid-Open Publication No. 2002-507496. This oil pump mechanism comprises a worm fitted on the crankshaft by insertion in a freely rotatable manner and connected to the clutch drum so as to be integrally rotated with the clutch drum, a driven gear engaged with the worm, and a movable pumping element for pumping the lubricating oil.
There is the need for providing an oil pump mechanism with a worm structure capable of assuring adequate slidability relative to the crankshaft and sufficient strength in a gear tooth region, while facilitating reduction in production cost.